Work begins on Hubble's replacement

Work has begun on the primary mirror of the James Webb Space Telescope at a new, state-of-the-art facility in Alabama. The mirror pieces will be shaped, but not polished, in a $1.2m, 20,000 square foot facility opened this month.

The primary mirror is 6.5m in diameter, and composed of 18 hexagonal pieces. Each of these 1.3m pieces will be made of beryllium. Beryllium tolerates extremely low temperatures very well, deforming much less than glass would in the same conditions. It is also extremely light, and the mirror will have less mass than Hubble's, much smaller, primary mirror.

The mirror will be machined to an extremely high degree of precision. Variations of no more than four thousandths of an inch are considered acceptable.

Four companies are involved in manufacturing the mirrors. First, Brush Wellman compressed beryllium into so-called blanks. Axsys then machines the blanks into their correct shapes. This process reduces the mass of the components by 92 per cent: the final segments will weigh just 21kg. Next, Tinsley Laboratories will grind and polish the mirrors and finally Ball builds the telescope. The whole process will take around four and a half years.

The James Webb Space Telescope is scheduled to launch in 2011. It will hang in space at the Lagrange-2 point of balance between the gravitational influences of the Earth and the sun. Astronomers will use the telescope to look back in the infra-red to the youngest days of the universe in the hope of finding out more about the birth of galaxies and stars, and gaining a better understanding of the size and shape of the universe. ®